1. Field of the Invention
The invention relates to a bilateral hearing aid system comprising at least one bone conduction output transducer.
2. Description of Related Art
Examples of bone conduction hearing aid systems are described in U.S. Patent Application Publications 2009/0245553 A1 and 2009/0247810 A1.
Bone conduction hearing aids are used by patients who cannot benefit from electro-acoustic hearing aids. Most of them are suffering from malformed ears, conductive hearing loss or single-sided deafness.
In general, bone conduction hearing aids use a mechanical transducer coupled to the skull to directly transfer sound vibrations through the bone to the cochlea, thereby bypassing the outer and the middle ear.
In case of non-implanted devices the transducer may be incorporated in a BTE (Behind The Ear) housing or an ITE (In the Ear) shell, having direct contact to the skull with the skin in-between, or it may be coupled to the skull using a head belt or an eyeglass adapter, or it may be coupled to the teeth.
In bone-anchored devices, surgically implanted abutments in the skull are used to achieve an improved coupling between the transducer and the skull. Such abutments may be magnets offering a strong transcutaneous magnetic coupling with the externally located transducer, or they may be designed as a percutaneous “screw” on which the transducer is sitting.
Usually the transducer forms part or is connected to an external sound processor, which typically is a BTE- or ITE-type device comprising one or more microphones, a signal processing and amplification unit and a driver for the transducer. The sound processor device is usually placed close to the ear to provide the most natural sound pick up position for the microphones. The transducer may be integrated in the sound processor housing or it may be a separate element connected by wire or by a wireless radio link to the sound processor.
It is generally desirable to fit hearing aids bilaterally in order to achieve the well-known advantages of binaural hearing in terms of speech understanding, sound quality and spatial hearing.
However, the benefit of bilateral fittings is limited in case of bone conduction hearing aids. The reason is that the interaural time differences (ITD) and interaural leveled differences (ILD) cues are disturbed due to the strong transcranial cross-talk effect of bone conduction. Bone-conducted vibrations reach the contralateral cochlea with an average attenuation of just about 10 dB compared to the ipsilateral cochlea. By contrast, for air conduction, i.e. electro-acoustic, hearing aids, the interaural attenuation typically is more than 50 dB. Hence, in case of bone conduction there is an unnatural interference of the sound coming from the ipsilateral transducer and the contralateral transducer. The result are deteriorated ITDs and ILDs, so that the benefit of binaural hearing is quite small compared to what could be expected is the cochleae received proper stimuli.
International Patent Application Publication WO 2009/101622 A2 relates to a sound system for reproducing recorded sound, comprising several loudspeakers and bone conduction speakers to be located at the right side and the left side of a user's head. It is mentioned that transcranial cross talking occurs with the use of bone conduction speakers, and a theoretical analysis of this effect is described. It is also mentioned that interesting effects can be achieved by controlling such cross-talking effect.
The article “Head-related two-channel stereophony with loudspeaker reproduction”, by P. Damaske, JASA, Vol 50, 1971 relates to cross-talk compensation techniques for virtual acoustic imaging with two free-field loudspeakers.